Fuel cells are envisioned as systems for supplying electrical power to mass-produced automotive vehicles in the future, and for many other applications. A fuel cell is an electrochemical device that converts chemical energy directly into electrical energy. Dihydrogen is used as fuel in fuel cells. The dihydrogen is oxidized on an electrode of the cell and dioxygen from the air is reduced on another electrode of the cell. The chemical reaction produces water. The great advantage of fuel cells is that they do not emit atmospheric pollutants at the point where the electricity is generated.
One of the major difficulties facing the development of such fuel cells lies in the synthesis and supply of dihydrogen. On earth, hydrogen exists in large amounts only combined with oxygen (in the form of water), with sulfur (hydrogen sulfide), with nitrogen (ammonia) or with carbon (fossil fuels such as natural gas or crude oils). Therefore, the production of dihydrogen either requires fossil fuels to be consumed, or substantial amounts of cheap energy to be available in order to obtain it via thermal or electrochemical decomposition of water.
The most common process for producing hydrogen from water thus consists in applying the principle of electrolysis. To carry out such processes, electrolyzers equipped with a proton-exchange membrane (PEM) are known. In such electrolyzers, an anode and a cathode are fixed on either side of the proton-exchange membrane (to form a membrane/electrodes assembly) and brought into contact with water. A potential difference is applied between the anode and the cathode. Thus, oxygen is produced at the anode by oxidation of the water. The oxidation at the anode also generates H+ ions that pass through the proton-exchange membrane to the cathode, electrons being supplied to the cathode by the electrical power supply. At the cathode, the H+ ions are reduced to generate dihydrogen.
In practice, such an electrolyzer generally comprises supply plates placed on either side of the membrane/electrodes assembly. Current collectors are placed between the supply plates and the membrane/electrodes assembly.
Such an electrolysis device is subject to undesirable effects. An issue for such a proton-exchange membrane electrolyzer is to increase its efficiency and its lifetime, to decrease its manufacturing cost and to guarantee a high level of safety. These parameters are highly dependent on the manufacturing process of the electrodes.
In a first type of process for manufacturing an electrode, catalytic ink is deposited in a layer on the current collectors. On the anode side, high overvoltages are required for electrooxidation of water. Thus, the anodic potential of the electrolyzer is in general very high (>1.6 VSHE). The use of carbon-containing materials and especially of diffusing layers made of carbon (felt, paper, carbon fabrics) is thus impossible (corrosion into CO2), whereas such materials are widely used on the cathode side. The current collectors on the anode side generally take the form of sintered parts made of porous titanium or of titanium meshes.
Depositing the electrocatalytic ink on such collectors has the major drawback of decreasing their porosity to water, thus limiting the transport of water to the catalyst. In addition, some of the catalyst material does not participate in the catalytic reaction (and therefore the amount thereof is unnecessarily large) because it is located in pores in the current collector. Also, the electrode/membrane interface is relatively poor because the effective surface area of the electrode is small. Thus, the performance of the electrolyzer is somewhat limited. Furthermore, locating the catalyst material in the pores of the current collector makes recycling the catalyst difficult. This is because it is particularly difficult to separate noble metals from the electrodes and current collectors. Moreover, such current collectors generally require precise and expensive machining operations to produce.
To solve certain of these drawbacks, a second type of manufacturing process involves depositing the electrocatalytic ink directly onto the proton-exchange membrane, so as to form a layer forming an electrode.
In such a process, the membrane absorbs moisture during deposition of the ink and thus swells and deforms. Next, contraction during drying also causes deformation. These deformations are not negligible and generate mechanical stresses in deposits, which may lead to cracks on the formed electroactive layer. Such cracks decrease the electronic percolation of the electrode and thus decrease its electrical conductivity. In addition, the cracks may decrease cohesion between the electrode and the membrane. Moreover, in operation, the membrane is completely submerged in water, maximizing its degree of swelling. The mechanical stresses at the interface between the electrode and the membrane are thus maximized, inducing additional deterioration of the electrode. This deterioration of the electrode decreases the energy efficiency of the electrolyzer and its lifetime.
Another problem with forming an electrode by catalytic deposition on the membrane is the damage caused to this membrane by solvents present in the ink (ethanol or isopropanol for example). On the one hand, the solvents increase the permeability of the membrane to gases. Some of the gases produced at the anode and at the cathode thus diffuse through the proton-exchange membrane. Not only does this cause problems with the purity of the gases produced, but it also causes safety problems. Specifically, the proportion of hydrogen in the oxygen must absolutely not exceed 4%, such a proportion being the lower explosive limit of hydrogen in oxygen. On the other hand, damage of the membrane by the solvents decreases its lifetime.